The present invention relates to a process for producing a silicon carbide single crystal and a production apparatus therefor, in which a silicon material is reacted with a carbon material to yield a silicon carbide single crystal, and more specifically, to a process for producing a silicon carbide single crystal and a production apparatus therefor, in which a melted or vaporized silicon material is introduced into a carbon material in order to generate a silicon carbide gas, which is then caused to reach a silicon carbide seed crystal substrate to thereby grow a silicon carbide single crystal.
Since silicon carbide is a substance having a hardness close to that of diamond and considerably high thermal and chemical stability, and serves as a semiconductor material having a wide energy band gap (about 3 eV), silicon carbide has conventionally been used as a polishing material, a refractory material, a heat generating material, or the like, and is expected to be used as a material for elements having high environmental resistance that are usable at elevated temperature, radiation resistance elements, power elements for electrical power control, and short-wavelength light-emitting elements.
A sublimation method is generally used in order to produce a silicon carbide single crystal (see, for example, Japanese Kohyo (PCT) Patent Publication No. 3-501118).
FIG. 8 is a sectional view showing a conventional apparatus for producing a silicon carbide single crystal, in which a sublimation method is employed. In FIG. 8, reference numeral 1 denotes a hollow crucible which is made of graphite and is composed of a crucible body 2 having a closed-bottomed cylindrical shape, and a lid plate 3 removably disposed on the upper end of the crucible body 2. A silicon carbide material 4 in the form of a powder is stored at the bottom of the crucible body 2, and a silicon carbide seed crystal substrate 5 is attached to the lower surface of the lid plate 3. Heating and heat-retaining heaters 6 and 7 are disposed outside the crucible 1, and temperature control is performed such that the temperature of the silicon carbide seed crystal substrate 5 is lower than that of the silicon carbide material 4.
When a silicon carbide single crystal is produced by use of the above-described apparatus, a predetermined amount of the powdery silicon carbide material 4 is placed at the bottom of the crucible body 2, and the silicon carbide seed crystal substrate 5 is attached to the lower surface of the lid plate 3, which is then fixed to the crucible body 2. Subsequently, the interior of the crucible 1 is filled with an inert gas such as Ar to create an inert gas atmosphere, and the silicon carbide material 4 and the silicon carbide seed crystal substrate 5 are heated to respective desired temperatures by use of heaters 6 and 7.
The powdery silicon carbide material 4 decomposes and sublimes due to heat and rises in the form of silicon carbide gas. The silicon carbide gas reaches the silicon carbide seed crystal substrate 5 maintained within a growth temperature region, so that a silicon carbide single crystal 8 is grown epitaxially.
A method in which a silicon carbide single crystal is grown through utilization of reaction between vaporized silicon and solid carbon is also used.
FIG. 9 is a sectional view showing a conventional apparatus for producing a silicon carbide single crystal through utilization of reaction between silicon and carbon. In FIG. 9, reference numeral 11 denotes a silicon material stored at the bottom of a crucible body 2; and reference numeral 12 denotes a carbon material which is disposed in the crucible body 2 to be located above the silicon material 11 and which reacts with vaporized silicon gas.
The carbon material 12 is composed of a carbon plate 13 having a large number of through holes 13a, and a powdery/granular carbon material 14 charged on the carbon plate 13.
When a silicon carbide single crystal is produced by use of the above-described apparatus, a predetermined amount of the silicon carbide material 11 in the form of granules or powder is placed at the bottom of the crucible body 2, and a silicon carbide seed crystal substrate 5 is attached to the lower surface of a lid plate 3, which is then fixed to the crucible body 2. Subsequently, the interior of the crucible 1 is depressurized. The silicon material 11 is heated by heater 7 such that the silicon material 11 is melted and vaporized, and the silicon carbide seed crystal substrate 5 is heated by heater 6 such that the silicon carbide seed crystal substrate 5 is maintained at a temperature suitable for growth of a silicon carbide single crystal.
Upon being heated, the silicon material 11 is melted and vaporized, so that the silicon material 11 rises in the form of silicon gas and passes through the through holes 13a of the carbon plate 13 and the powdery/granular carbon material 14. While passing through the carbon material 12, the silicon gas reacts with the carbon material 12 to generate a silicon carbide gas. The silicon carbide gas reaches the silicon carbide seed crystal substrate 5 maintained within a growth temperature region, so that a silicon carbide single crystal 8 is grown epitaxially.
However, these conventional methods have a drawback in that silicon gas or molten silicon reacts with graphite, which is the material of the crucible, thereby damaging the crucible. In the worst case, a hole is formed in the crucible, with the result that silicon gas and silicon melt accommodated within the crucible may leak outside, leading to a situation in which the production must be stopped. In this case, the damaged crucible is replaced with a new crucible so that a single crystal can again be grown. However, the expense of purchase of the new crucible for replacement and time loss caused by the operation of heating the new crucible to a temperature required for crystal growth lead to various problems, such as an increase in production cost and a decrease in productivity.
Especially, in the conventional sublimation method, when powdery silicon carbide material 4 is heated, not only SiC, but also Si, Si2C, and SiC2 are generated in the form of decomposition/sublimation gases. Since the amount of the silicon component in the sublimation gas as a whole becomes equimolar or greater with respect to the amount of the carbon component, the composition of the powdery silicon carbide material 4 changes gradually during the course of the sublimation process such that the carbon content becomes in excess. Therefore, when the powdery silicon carbide material 4 sublimes, the partial pressures of the above-described decomposition/sublimation gases change with time, so that crystal defects tend to be generated to a greater extent in a silicon carbide single crystal, with the result that the crystallinity of the silicon carbide single crystal obtained decreases.
Meanwhile, in the conventional single crystal growth method utilizing the reaction between silicon and carbon, the proportions of the respective components of a reaction gas, such as SiC, Si, Si2C, and SiC2, are difficult to accurately control because of difficulty in controlling, to a predetermined amount, the quantity of silicon gas supplied to the carbon material. As a result, generation of crystal defects occurs easily during crystal growth, resulting in decreased crystallinity of the silicon carbide single crystal obtained.
The reason why the amount of silicon gas supplied to the carbon material is difficult to control is as follows. The amount of silicon gas supplied is controlled through control of the amount of silicon material vaporized by the temperature of silicon material. However, since the amount of the silicon material changes with time and the temperature inside the crucible changes, the amount of silicon gas supplied is difficult to control to a constant level.
In order to grow a single crystal of high quality, the temperatures of the carbon material and the silicon carbide seed crystal substrate must be controlled to fall within predetermined temperature ranges. However, when temperature control is performed in order to maintain a constant amount of vaporized silicon material, the temperatures of the carbon material and the silicon carbide seed crystal substrate are affected thereby. Therefore, controlling only the supply amount of the silicon gas to a constant level is difficult in practice.
The present invention was accomplished in view of the foregoing situation, and an object of the present invention is to provide a process for producing a silicon carbide single crystal and a production apparatus therefor which enables continuous production of a silicon carbide single crystal, which has a reduced density and dispersion of crystal defects in a growth direction, no lattice distortion, a large diameter, and constant quality under stable conditions.
In order to solve the above-described problems, the present invention provides process for producing a silicon carbide single crystal and a production apparatus therefor as described below.
That is, in a first embodiment, the invention provides a process for producing a silicon carbide single crystal comprising introducing a melted or vaporized silicon material from the outside of a reaction system into a carbon material heated to a temperature equal to or higher than a temperature at which the silicon material vaporizes; and causing a reaction gas containing silicon gas and silicon carbide gas generated by a reaction between the carbon material and the silicon material to reach a silicon carbide seed crystal substrate which is held at a temperature lower than that of the carbon material, so that a silicon carbide single crystal grows the silicon carbide seed crystal substrate.
In a second embodiment, the invention provides a process for producing a silicon carbide single crystal according to the first embodiment of the process for producing a silicon carbide single crystal, wherein the carbon material is in the form of a carbon layer filled with powdery/granular carbon material, a porous carbon structure, a carbon plate having many through holes, or any combination of these.
Also in a third embodiment, the invention provides a process for producing a silicon carbide single crystal according to the second embodiment of the process for producing a silicon carbide single crystal, wherein that the powdery/granular carbon material is carbon granules having an average grain size of 100 xcexcm to 5 mm.
In a fourth embodiment, the invention provides a process for producing a silicon carbide single crystal according to the first, second and third embodiment of the process for producing a silicon carbide single crystal, wherein the carbon material contains silicon carbide.
In a fifth embodiment, the invention provides a process for producing a silicon carbide single crystal of the process for producing a silicon carbide single crystal according to any one of embodiments 1 to 4, wherein the temperature range of the silicon carbide seed crystal substrate is 1500-2500xc2x0 C., the temperature range of the carbon material is set such that the temperature of the carbon material is higher than that of the silicon carbide seed crystal substrate, and the temperature difference between the carbon material and the silicon carbide seed crystal substrate does not exceed 400xc2x0 C.
In a sixth embodiment, the invention provides a process for producing a silicon carbide single crystal according to embodiments 1 to 5 of the process for producing a silicon carbide single crystal, wherein the silicon material and/or the carbon material is supplied continuously or intermittently.
In a seventh embodiment, the invention provides a process for producing a silicon carbide single crystal according to embodiments 1 to 6 of the process for producing a silicon carbide single crystal, wherein the process includes growing the silicon carbide single crystal simultaneous with a discharging of the reaction gas to the outside of the reaction system and/or moving the grown silicon carbide single crystal to the outside of the reaction system in a volume corresponding to the volume of the silicon material introduced.
The invention also in an eighth embodiment provides an apparatus for producing a silicon carbide single crystal comprising a crucible which stores a carbon material and has a top portion having a lower surface to which a silicon carbide seed crystal substrate is attached; heating means for elevating and maintaining the temperature of the crucible; a material container for storing a silicon material and for elevating and maintaining the temperature of the silicon material; and a silicon introduction tube for introducing from the material container into the crucible the silicon material in a molten or vaporized state.
In a ninth embodiment, the invention provides an apparatus for producing a silicon carbide single crystal according to the apparatus for producing a silicon carbide single crystal according to embodiment 8, wherein at least the inner wall of the crucible is formed of silicon carbide.
The invention in a tenth embodiment provides an apparatus for producing a silicon carbide single crystal according to the apparatus for producing a silicon carbide single crystal according to embodiment 8, wherein at least the inner wall of the silicon introduction tube is formed of silicon carbide and/or a composite material thereof.
The invention in an eleventh embodiment provides an apparatus for producing a silicon carbide single crystal according to the apparatus for producing a silicon carbide single crystal according to embodiments 8, 9 or 10, wherein the carbon material is disposed at a plurality of stages provided along the flow direction of the silicon material.
In a twelfth embodiment, the invention provides an apparatus for producing a silicon carbide single crystal according to the apparatus for producing a silicon carbide single crystal according to embodiments 8, 9, 10 or 11, wherein the heating means comprises a first heating means for elevating and maintaining the temperature of the carbon material and a second heating means controlled independently of the first heating means and adapted to elevate and maintain the temperature of the silicon carbide seed crystal substrate.